Some Tasmanian schools are being forced to cut sports, art and music classes to make up for a shortfall in disability education funding, a peak body for principals has said.

State and independent schools are calling for the urgent introduction of a needs-based funding system.

Scottsdale mother Rebecca Kitchin's five-year-old son Xavier has an intellectual disability, issues with his gross motor skills and is incontinent.

Ms Kitchin said despite Xavier being on the Severe Disability Register, he is not receiving enough support at school.

"We've encountered times when there hasn't been an aid on hand to change him, so they have had to pool resources to allocate change times, so it just means he hasn't got that one-on-one support fulltime."

Xavier's school week is split between his local primary school and the Northern Support School, which is about one hour from his home.

"Due to not having the required support in the mainstream schooling we have had to do this a lot sooner," Ms Kitchin said.

Disability advocate Kristen Desmond said a huge government funding gap means Xavier's experience is not an isolated one.

"Kids are being forced into part-time enrolment or home schooling because the system simply cannot cope," she said.

In Tasmania an IQ-based system is used to allocate funding for students with disabilities.

State schools do not receive direct funding for students with IQs above 70, and Ms Desmond said this could include some students with autism, cerebral palsy and attention deficit disorder.

"What we are hearing is schools are pooling that resource and spreading it across students who are not funded, leaving everybody slowly worse off," Ms Desmond said.

Tasmanian Principals Association's (TPA) Malcolm Elliott said state schools receive state and federal funding and were having to dip into their budgets to pay for extra support, like integration aids and speech therapists.

"It might mean a cut to the health and wellbeing project, physical education, it might be a cut in the provision for art, it might mean a cut in the provision of music, or it might mean there is no provision for any of those things," he said.

Independent Schools Association (IST) of Tasmania's Tony Crehan said some independent schools, which typically receive about $5,000 in federal funding for each student with a disability, were forced to increase school fees.

"We have students with very high needs in the extensive and substantial categories, whose additional cost to the school to support the students might be upwards of $30,000, $40,000, $50,000," he said.

To address the funding shortfall, Ms Desmond, the TPA and IST want the state and federal governments to come through with a needs-based funding system.

"IQ is not a measure of the reality of the support that is required. We need to look at people's functionality, and what they need, what supports the need in order to function in a classroom," Ms Desmond said.

In a statement, Federal Education Minister Simon Birmingham said data collected so far to better inform funding distribution was not yet robust enough.

Mr Birmingham said the "results are frustrating and must be disappointing for families of children with disability".

The Hodgman Liberal Government has invested $12 million over four years to implement the recommendations of the Ministerial Taskforce's report into Education for Students with Disability.